Milk of magnesia is magnesium hydroxide suspended in water — the white, chalky liquid used as a laxative for constipation and as an antacid for indigestion and heartburn. The same bottle does both jobs; the difference is the dose.
What is Milk Of Magnesia?
Milk of magnesia is magnesium hydroxide in water, sold as a suspension in pharmacies and supermarkets. It is a general-sale product rather than a registered scheduled medicine, which is why it does not appear in the South African Medicine Price Registry. That absence is unremarkable — it says nothing about quality.
Read your own bottle, since the concentration and dosing instructions differ between products.
Magnesium hydroxide is an alkaline salt, and it works in two entirely different ways depending on how much you take:
At a small dose, it is an antacid. Magnesium hydroxide neutralises stomach acid on contact, relieving heartburn and indigestion within minutes.
At a larger dose, it is an osmotic laxative. Magnesium is poorly absorbed from the gut, so it stays in the bowel and draws water in by osmosis. The extra water softens the stool and increases bulk, which stimulates the bowel to move — usually within 30 minutes to six hours.
That dual action is why one product appears in two different sections of a pharmacy, and it is worth knowing: take an antacid dose and you may get a laxative effect anyway, which is the commonest surprise with it.
Shake the bottle well. The magnesium settles, and an unshaken bottle gives an inconsistent dose.
What is Milk Of Magnesia commonly used for?
As a laxative:
- Occasional constipation, in adults and — at appropriate doses — children.
- Constipation from medicines, particularly opioids, iron or antidepressants.
- Constipation in pregnancy, where it is one of the options, though bulk-forming laxatives are usually tried first.
- Bowel preparation, in some contexts on medical direction.
As an antacid:
- Heartburn and acid indigestion.
- Acid reflux, for short-term relief.
- Stomach discomfort after eating.
It is for occasional use. That is not a formality — regular laxative use has its own consequences, and it also means constipation is being managed rather than addressed.
What actually addresses constipation is less interesting and more effective: more fibre (fruit, vegetables, wholegrains, legumes), more fluid, regular activity, and not ignoring the urge to go. Where a laxative is needed regularly, a bulk-forming agent such as ispaghula or an osmotic such as macrogol or lactulose is generally preferred for long-term use.
Constipation with a change in bowel habit needs assessment, not a laxative. A persistent change lasting weeks, blood in the stool, unexplained weight loss, or constipation alternating with diarrhoea are all reasons to see a doctor.
How Milk Of Magnesia fits into this medicine category
| Group | Examples | How it works |
|---|---|---|
| Osmotic laxatives | Milk of magnesia, lactulose, macrogol | Draw water into the bowel |
| Bulk-forming laxatives | Ispaghula, methylcellulose | Add bulk; closest to dietary fibre |
| Stimulant laxatives | Senna, bisacodyl | Stimulate bowel contractions; not for long-term daily use |
| Stool softeners | Docusate, liquid paraffin | Soften stool |
| Antacids | Milk of magnesia, aluminium salts, Eno | Neutralise acid on contact |
| Acid reducers | Famotidine, omeprazole | Reduce acid production; slower, longer-lasting |
| Antispasmodics | Hyoscine (Hyospasmol) | Cramping, not acid or constipation |
Magnesium-containing antacids tend to loosen the stool; aluminium-containing ones tend to constipate, which is why many antacid products combine both. See the digestive and stomach medicines hub, which also covers metoclopramide (Adco-Contromet).
What to check before using Milk Of Magnesia
Kidney function is the critical check. Magnesium is cleared by the kidneys, and in kidney impairment it accumulates — causing hypermagnesaemia, which lowers blood pressure, slows the heart, causes muscle weakness and confusion, and can be fatal. Do not use magnesium-containing products regularly if you have kidney disease without medical advice.
Also speak to a pharmacist or doctor if you:
- Have kidney disease or reduced kidney function, or are elderly — kidney function declines with age even without a diagnosis.
- Have heart block or a heart rhythm problem.
- Have abdominal pain, nausea or vomiting of unknown cause — never take a laxative for undiagnosed abdominal pain, since bowel obstruction is made worse by it.
- Have a bowel obstruction, or have had bowel surgery.
- Are pregnant or breastfeeding.
- Are treating a child — use the dose for their age, and ask a pharmacist.
- Take other medicines, since magnesium reduces the absorption of many. Separate it by about two hours from tetracycline and quinolone antibiotics, iron, thyroid medicine, bisphosphonates, digoxin and HIV medicines such as dolutegravir.
Practical points:
- Shake well before each dose.
- Take the laxative dose with plenty of water, and expect an effect within 30 minutes to six hours.
- Take the antacid dose after meals and at bedtime, when acid symptoms occur.
- Do not use it as a laxative for more than a few days without advice.
- Do not take it if you have abdominal pain of unknown cause.
Possible cautions and when to ask a pharmacist or doctor
Common effects are diarrhoea, loose stools, abdominal cramping and a chalky taste. Diarrhoea usually means the dose was too high.
Get emergency help for signs of magnesium excess, which matter most in kidney impairment:
- Marked muscle weakness, or feeling floppy
- Confusion or drowsiness
- A slow or irregular heartbeat, or very low blood pressure
- Difficulty breathing
Also get emergency help for:
- Severe abdominal pain, a swollen tense abdomen, or vomiting with no bowel movement — possible obstruction.
- Vomiting blood, or black tarry stools.
- Chest pain with breathlessness or sweating — heart attack pain is frequently mistaken for indigestion, and an antacid is frequently taken instead of calling for help.
- Swelling of the face, lips or tongue, or difficulty breathing.
Contact your doctor if:
- Constipation lasts more than a week, or you need a laxative most days.
- There is blood in the stool, or the stool is black or tarry.
- You have a change in bowel habit lasting more than a few weeks.
- There is unexplained weight loss, persistent abdominal pain, or constipation alternating with diarrhoea.
- You need an antacid most days, which suggests reflux needing proper treatment.
- You have kidney disease and have been using it regularly.
- A child is constipated persistently — that needs assessment rather than repeated laxatives.
Related medicine guides
Frequently asked questions
What is milk of magnesia used for? Two things: as a laxative for occasional constipation, and as an antacid for heartburn and indigestion. The difference is the dose.
How long does it take to work as a laxative? Usually between 30 minutes and six hours.
Why does it sometimes cause diarrhoea when I take it for heartburn? Because the same ingredient is a laxative at higher doses. If the antacid dose loosens your stool, take less or use a different antacid.
Can I use it every day? Not without advice. It is for occasional use — regular constipation needs dietary changes or a laxative better suited to long-term use.
Can I take it with my other medicines? Separate them by about two hours. Magnesium reduces absorption of several, including some antibiotics, iron, thyroid medicine and HIV treatment.
Can I take it with kidney disease? Not without medical advice. Magnesium accumulates when the kidneys are impaired and can become dangerous.
Do I need to shake it? Yes, well, before every dose — the magnesium settles and an unshaken bottle doses unevenly.
Can I give it to a child? At the dose for their age, and ask a pharmacist. Persistent constipation in a child needs assessment.